Why does realloc cause a segfault here?

This is a discussion on Why does realloc cause a segfault here? within the C Programming forums, part of the General Programming Boards category; I'm looking at this code, but I can't seem to figure out why it's segfaulting. The purpose of the function ...

  1. #1
    Registered User
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    Jun 2008
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    54

    Why does realloc cause a segfault here?

    I'm looking at this code, but I can't seem to figure out why it's segfaulting.
    The purpose of the function is to put each period separated word (token) into a list that grows as large as necessary.
    [Edit: Oh, uh, this is C89.]
    Code:
    #include <stdlib.h>
    static int getdots(char* input, char** keylist) {
    	keylist = calloc(1, sizeof(char*));
    
    	int i = 0;
    	keylist[i] = malloc(512* sizeof (char));
    	memset(keylist[i],'\0', 512*sizeof(char));
    	char* p = input, *c=keylist[0];
    	while (*p!='\0') {
    
    		while(c && p)
    		if(*p!='.')
    		*c++=*p++;
    		else
    		break;
    
    		if(*p=='.')
    		{
    
    			i++;
    			keylist=realloc(keylist, sizeof(char*)*(i+1));
    			keylist[i]=malloc(512*sizeof(char));
    			c=keylist[i];
    			memset(keylist[i],'\0', 512*sizeof(char));
    		}
    		p++;
    	}
    	return 0;
    }
    
    int main() {
    
    	char input[512] = { "hello.helm.happy\0" };
    	char** keylist = NULL;
    	getdots((char*) &input, keylist);
    	if (!keylist) {
    		printf("Nothing happened!\r\n");
    		return 0;
    	}
    	int i = 0;
    	for (i = 0; keylist[i]; i++)
    		printf("%s\r\n", keylist[i]);
    	if (keylist != NULL)
    		free(keylist);
    	return 0;
    
    }
    Last edited by Boxknife; 05-06-2009 at 04:13 PM.

  2. #2
    ATH0 quzah's Avatar
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    Code:
    while(c && p)
    The chances of these actually pointing to NULL are very slim. You should be checking to see that you don't run past your line length, or that they contain '\0'. A pointer pointing at a character containing '\0' will pass a NULL check.
    Code:
    #include<stdio.h>
    int main( void )
    {
        char c = '\0';
        char *p = &c;
    
        if( p )
        {
            printf( "Not null.\n" );
        }
    
        return 0;
    }
    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
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    54

    Fixed program

    It's now working.
    For whatever it's worth, here is the fixed program:
    Code:
    #include <stdlib.h>
    #include <ctype.h>
    #include <stdio.h>
    #include <string.h>
    #define LOWER(c)   (((c)>='A'  && (c) <= 'Z') ? ((c)+('a'-'A')) : (c))
    static char** getdots(char* input, char** keylist) {
    
    	int i=0;
    	keylist[i]=malloc(512*sizeof(char));
    	memset(keylist[i],'\0', 512*sizeof(char));
    	char* p = input, *c=keylist[i];
    	while (*p!='\0') {
    
    		while(*p!='.' && *p!='\0')
    		*c++=*p++;
    
    		if(*p=='\0')
    		{
    		
    		return keylist;
    		}
    		if(*p=='.')
    		{
    		i++;
    		keylist= realloc(keylist, sizeof(char*)*(i+1));
    		if(!keylist){
    		printf("Keylist broken.\r\n");
    		return NULL;
    		}
    		keylist[i]=malloc(512*sizeof(char));
    		c=keylist[i];
    		memset(keylist[i],'\0', 512*sizeof(char));
    		}
    		p++;
    	}
    	return keylist;
    }
    
    int isname(const char *str, const char *namelist) {
    	const char *curname, *curstr;
    	curname = namelist;
    	for (;;) {
    		for (curstr = str;; curstr++, curname++) {
    			if (!*curstr && !isalpha(*curname))
    				return (1);
    
    			if (!*curname)
    				return (0);
    
    			if (!*curstr || *curname == ' ' || *curname == '-' || *curname == ',')
    				break;
    
    			if (LOWER(*curstr) != LOWER(*curname))
    				break;
    		}
    
    		/* skip to next name */
    
    		for (;isalpha(*curname); curname++)
    			;
    		if (!*curname)
    			return (0);
    		curname++; /* first char of new name */
    	}
    }
    
    
    int main()
    {
    
    char input[512]={"big.r.helm.yay\0"};
    char sdesc[512]={"a big, red helm\0"};
    char** keylist=NULL;
    keylist=calloc(1,sizeof(char*));
    
    keylist=getdots((char*) &input, keylist);
    if(!keylist)
    {
    printf("Nothing happened!\r\n");
    return 0;
    }
    int i=0;
    for(i=0;keylist[i];i++)
    {
    printf("%s\r\n",keylist[i]);
    if(isname(keylist[i], sdesc))
    printf("%s is in the list.\r\n", keylist[i]);
    else
    printf("%s is not in the list.\r\n", keylist[i]);
    }
    
    if(keylist)
    free(keylist);
    
    return 0;
    
    
    
    
    }
    Last edited by Boxknife; 05-06-2009 at 07:41 PM.

  4. #4
    and the hat of wrongness Salem's Avatar
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    Indentation needs work, and there's still a couple of bugs....
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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